Homogeneous or non-supported alumoxane metallocene catalysts are known for their high catalytic activity in olefin polymerizations. Under polymerization conditions where polymer is formed as solid particles, these homogeneous (soluble) catalysts form deposits of polymer on reactor walls and stirrers, which deposits should be removed frequently as they prevent an efficient heat-exchange, necessary for cooling the reactor contents, and cause excessive wear of the moving parts. The polymers produced by these soluble catalysts further have a low bulk density which limits the commercial utility of both the polymer and the process. In order to solve these problems, several supported alumoxane metallocene catalysts have been proposed for use in particle forming polymerization processes.
U.S. Pat. No. 5,057,475 describes a supported metallocene alumoxane catalyst wherein the alumoxane can be a commercial alumoxane, or an alumoxane generated in situ on the solid support, for example, by the addition of a trialkylaluminum compound to a water-containing support, such as by addition of trimethylaluminum to a water containing silica. In the preferred methods of U.S. Pat. No. 5,057,475, the metallocene component and the alumoxane (which previously may have been combined with a modifier compound) are combined in a first step in a suitable solvent. In a subsequent step, this solution is contacted with the support. Then, the solvent can be removed, typically by applying a vacuum. The solution may be heated in order to aid in the removal of the solvent. In an alternative method, an undehydrated silica gel is added to a solution of trialkylaluminum to produce an alumoxane which is deposited onto the surface of the silica gel particles. Then, the solvent is removed and the residual solids are dried to a free-flowing powder. In typical examples, dried silica is slurried with an alumoxane in toluene, filtered, washed with pentane, and then dried under vacuum. The metallocene compound is typically combined with an alumoxane in toluene or heptane, which solution subsequently is combined with the pretreated silica. Finally, the toluene or heptane is removed under vacuum to recover the supported catalyst.
U.S. Pat. No. 5,026,797 describes treating a porous water-insoluble inorganic oxide particle support with an alumoxane in a solvent for the alumoxane, such as an aromatic hydrocarbon, followed by rinsing the treated support with an aromatic hydrocarbon solvent until no alumoxane is detected in the supernatant. Thus, it is said to be possible to adjust the amount of the aluminum atom of the alumoxane bonded onto the treated oxide support in the range of 2 to 10 percent by weight. Subsequently, the treated support is combined with a zirconium compound. The so-formed support material containing alumoxane and zirconium compound is used together with additional alumoxane in solution in a polymerization reaction.
U.S. Pat. No. 5,147,949 discloses supported metallocene alumoxane catalysts prepared by adding a water-impregnated catalyst support to a stirred solution of an aluminum trialkyl, and adding to the reaction product thereof a metallocene component.
U.S. Pat. No. 5,240,894 describes a method to produce a supported catalyst by forming a metallocene/alumoxane reaction solution, adding a porous carrier, evaporating the resulting slurry so as to remove residual solvent from the carrier, and optionally prepolymerizing the catalyst with olefinic monomer. A good polymer bulk density is only obtained using a prepolymerized supported catalyst.
U.S. Pat. No. 5,252,529 discloses solid catalysts for olefin polymerization comprising a particulate carrier containing at least one percent by weight of water, an alumoxane compound, and a metallocene compound. In the preparation of this catalyst, the reaction product of the particulate carrier and the alumoxane is separated from the diluent (toluene) by decantation or drying at reduced pressure.
European Patent No. Application No. 368,644 discloses a process for preparing a supported metallocene alumoxane catalyst wherein an undehydrated silica gel is added to a stirred solution of triethylaluminum, to which reaction mixture is added a solution of a metallocene to which trimethylaluminum has been added. Following the addition of the trimethylaluminum treated metallocene to the triethylaluminum treated silica gel solids, the catalyst is dried to a free-flowing powder. Drying of the catalyst may be done by filtration or evaporation of solvent at a temperature up to about 85.degree. C.
European Patent Application No. 323,716 discloses a process for preparing a supported metallocene alumoxane catalyst by adding undehydrated silica gel to a stirred solution of an aluminum trialkyl, adding a metallocene to the reacted mixture, removing the solvent, and drying the solids to a free-flowing powder. After the metallocene has been added, the solvent is removed and the residual solids are dried at a temperature of up to about 85.degree. C.
European Patent Application No. 523,416 describes a supported catalyst component for olefin polymerization prepared from an inorganic support and a metallocene. The metallocene and support are intensively mixed in a solvent. Preferably, the catalyst component thus obtained is extracted in a suitable solvent, such as toluene, to remove metallocene which is not fixed. Subsequently, alumoxane can be added as a cocatalyst.
European Patent Application No. 567,952 describes a supported polymerization catalyst comprising the reaction product of a supported organoaluminum compound and a metallocene catalyst compound. This supported catalyst is prepared by combining trimethylaluminum with a previously dried support material in an aliphatic inert suspension medium, to which water is added. This suspension can be used as such or can be filtered and the solids thus obtained can be resuspended in an aliphatic inert suspension medium, and then combined with the metallocene compound. When the reaction is complete, the supernatant solution is separated off and the solid which remains is washed once to five times with an inert suspending medium, such as toluene, n-decane, diesel oil or dichloromethane.
It would be desirable to provide a supported catalyst component, a supported catalyst, and a polymerization process that prevents or substantially reduces the problem of reactor fouling, including formation of polymer deposits on reactor walls and on the agitator in the reactor, especially in gas phase polymerization or slurry polymerization processes. Further, it is preferred that polymer products produced by gas phase polymerization or slurry polymerization processes are in free-flowing form and, advantageously, have high bulk densities.